The drum towers are four:—The north-west or black tower; the south-west or Astragun Tower; the south-east or Margaret Dun Tower; and the north-east tower. All are of one date, and nearly of one pattern,—cylindrical, 30 feet diameter, with walls 9 feet thick. They are 44 feet high, with a battlement, and each has a well-stair at the gorge, terminating above in a square turret, the top of which is 53 feet high. Each contains a _souterrain_, and three stages of circular chambers, looped outwards, usually with three openings. There are also the usual chambers in the walls, and doors opening upon the ramparts of the curtains. There are also some points of difference. The chambers of the south-west tower are vaulted; the rest have timber floors. In this tower the _souterrain_ is entered by a long gallery in the wall, common to the south curtain and the kitchen, opening from the porter’s lodge. This is the “porter’s prison,” and possibly the vaulting was to allow the whole tower to be used for this purpose. The north-west tower is cylindrical below, but above it passes into a sort of heart-shaped plan, presenting towards the gorge a double bow, with a flat recess between. These two towers cap the angle of their curtains, and are engaged in about one-fifth of their circumference. The north-east and south-east towers are placed, not at the angles, but on the faces of the north and south curtains, close to their east ends, and on the edge of the river cliff. Besides their junction with the curtains of the inner ward, they are connected north and south with that part of the curtain of the outer ward which is built along the edge of the cliff. The south-east tower is closely connected with the chapel tower, and has also a door opening upon the curtain of the outer ward, leading to the great gatehouse. The curtains are four in number. That to the east is irregular, low, and weak; the cliff has been considered its defence. The other three are 6 feet thick, and 18 feet high to the top of the parapet. Along each is a rampart walk, having a parapet pierced with loops and a rere wall. The curtains enclose a quadrangle 80 yards square. The north curtain is pierced by a gateway, 6 feet wide, with a low drop arch, and the groove of a portcullis worked from the rampart; it is near the east end of the wall, and nearly opposite the northern gatehouse. This curtain is pierced by two loops, which seem to have been defended from the ground level. The west curtain has three loops, two of which opened from the kitchen. The south curtain is pierced near its centre by a gateway, 10 feet wide, with a low drop arch and portcullis groove. The wall has been thickened to give depth to the gateway, and possibly to allow of the superstruction of a low tower above it. This is the principal entrance to the inner, and opens towards the great gatehouse of the outer, ward.